Measuring the Solar System - the Story of the Transits of Venus

Introduction

In the 16th and 17th centuries our knowledge of the motions of
the planets rapidly improved. This was due to the work of several
outstanding astronomers and two brilliant mathematicians.

The astronomers were: Copernicus, who proposed that the
Earth moves around the Sun, Tycho Brahe, who made meticulous
naked eye observations of the planets, and Galileo, who first
utilised a telescope for astronomy.

The mathematicians were: Kepler, who showed that the planets
moved in elliptical orbits, and Sir Isaac Newton, who was able to
explain the relative positions and motions of the planets from a
simple application of his Universal Law of Gravitation.

Newton's theory was so comprehensive that it was felt by many
that the science of astronomy was now complete. Only one vital
piece of information was missing, the distance from the Earth to
the Sun, for, though Newton's theory was able to predict the
relative distances of all the planets from the Sun, it could not
predict the actual distance to any one of them. But, once the
distance to one planet was known, all others would follow. It was
as if the distance to Dublin from Armagh was known to be twice
the distance to Belfast, but neither distance was actually known
in miles.